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Alum-(K) (also known as Potassium Alum)

Discovered in 1875;   IMA status:  Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

KAl(SO₄)₂·12(H₂O)

Hydrated Potassium Aluminum Sulfate

Molecular Weight:

474.39 gm

Composition:

Potassium

8.24 %

K

9.93 %

K₂O

Aluminum

5.69 %

Al

10.75 %

Al₂O₃

Hydrogen

5.10 %

H

45.57 %

H₂O

Sulfur

13.52 %

S

33.75 %

SO₃

Oxygen

67.45 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Sulfates

Strunz 8th Ed. ID:

6/C.14-20

Nickel-Strunz 10th Ed. ID:

7.CC.20

7 : SULFATES (selenates, tellurates, chromates, molybdates, wolframates)
C : Sulfates (selenates, etc.) without additional anions, with H₂O
C : With medium-sized and large cations

Related to:

Alum Group. Sodium Alum - Zincovoltaite Series.

Members of Group:

Alum Group: Alum-(K), Alum-(Na), Lanmuchangite, Tschermigite

Varieties:

None

Synonyms:

Native Alum, Potassium Alum, Potash Alum, Tawas

Crystal Data

Crystallography:

Isometric - Diploidal

Crystal Habit:

Rarely as tiny octahedral crystals, if precipitated from pure water solution; cubic from alkaline solutions. Commonly stalactitic, columnar, granular, or massive efflorescences.

Twinning:

Very rare; on {111}

Physical Properties

Cleavage:

{111} Poor/Indistinct, Interrupted

Fracture:

Conchoidal

Tenacity:

Brittle

Moh's Hardness:

2.0 - 2.5

Density:

1.757 (g/cm³)

Luminescence:

None

Radioactivity:

Barely Detectable; GRapi = 124.32 (Gamma Ray American Petroleum Institute Units)

Other:

Soluble in water, taste sweetish and astringent; deliquesces at 91º C

Optical Properties

Color:

Colorless, White; Colorless in transmitted light

Transparency:

Transparent to Translucent

Luster:

Vitreous

Refractive Index:

1.456  Isotropic

Birefringence:

None (Isotropic); may be weakly anomalously birefringent and sectored

Dispersion:

n/a

Pleochroism:

None

Occurances

Geological Setting:

Formed from argillaceous rocks or coal containing oxidizing pyrite or marcasite; may be a fumarolic or solfataric precipitate.

Common Associations:

Alunogen, Gypsum, Epsomite, Melanterite, Pickeringite, Sulfur

Common Impurities:

None

Type Locality:

Possibly Alum Grotto, Porto Levante, Vulcano Island, Eolie Islands (Aeolian Islands), Lipari, Messina Province, Sicily, Italy

Year Discovered:

1875

View mineral photos:

Potassium Alum Mineral Photos and Locations

Unusual Gem Categories

Radioactive Gems, Water Soluble Gems

More Information

Mindat.org
Webmineral.com

Alum-(K) (also known as Potassium Alum) is a sulfate mineral that is very rarely available as a faceted gem. It is a very unusual mineral in that it is both barely radioactive and water soluble. It was originally named Potassium Alum but was renamed to Alum-(K) by Mineralogical Record, v39 (2008), p131 to indicate that it is the K-dominant member of the Alum Group. Alum-(K) is commonly used in water purification, leather tanning, and fireproofing textiles. It also has cosmetic uses as an ingredient in deodorant and as an aftershave treatment due to its astringent qualities.

Alum-(K) is named from the Latin word alumen for aluminum since it is the potassium double sulfate of aluminum and the -(K) suffix denotes that it is the K-dominant member of the Alum Group.

There are very few localities to find gem quality crystals of Alum-(K). A few of these are the El Desierto mine, San Pablo de Napa, Daniel Campos Province, Potosí Department, Bolivia; Alaunhöhle, near Dresden, Saxony, Germany; Alum mine, Alum District, Esmeralda County, Nevada, USA; Alum King Mine, Marysvale District (Marysvale Uranium area), Piute County, Utah, USA.

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